The Anti-Cruelty Society, where she and her employees were trained, holds 12 pet CPR certifications classes a year, and six or eight people complete the $50, three-hour courses, said Dr. Robyn Barbiers, the society's president. When Castro took the CPR training class she practiced on cat and dog mannequins.

Barbiers said typically pet owners took the classes, but now dog walkers outnumber the rest.

Barbiers was the lead veterinarian at Lincoln Park Zoo and an emergency care vet before that.

She said she's given CPR to all kinds of animals, including chimps, siamangs and a cantankerous lion named Cranky.

While in surgery for a uterus infection that turned necrotic, Cranky "decided she was going to stop breathing — and her heart stopped beating," Barbiers said.

But Barbiers successfully performed CPR, finished up the surgery, and Cranky went on to live another three years at Lincoln Park Zoo.

Another dog-walking company owner, Tony Schreck, said in the 12 years he has walked dogs he's never been in a CPR-necessary situation.

But Schreck has performed CPR twice on humans, he said. Although uncertified for both human and pet CPR, Schreck said he had just been in the "wrong place at the wrong time" and instincts took over. Both times the person ended up making it.

His company, Windy City Dog Walkers, has walked 5,000 dogs and does 30,000 walks a year, he said.

Not all dog-walking and pet-sitting companies require their walkers to have CPR certification.

Dog walker Al Olsen has 12 contracted employees and said the cost would be too much to require each of his walkers to get certified.

"You get to a certain point and have to take a look at the numbers," Olsen said. "It's going to cost $50 dollars for the class, and you have to pay them for their time. You're looking at $100 per person."

So, what does it take to revive an animal that stopped breathing? Watch the video to see Castro give DNAinfo.com Chicago a demonstration.